Re: QOS Bc Be and Tc

Hi Masroor,

Well the simplest and easiest way to think about it (whether with
policing/shaping) is that you are connected via a Fastethernet interface to
a provider (however most probably you won't have 100 Mbps bought from the
provider), and one thing about interfaces they are either ON or OFF. So you
can tell it please only send 2 Mbps or 3Mbps..Thus the only way to control
is to turn it ON/OFF in such a manner that you will be sending the rate you
already bought.

Thus the CIR is what you already agreed upon with the Provider. Bc is the
same as the CIR however within a smaller time frame Tc. Suppose for instance
you are given 2Mbps (CIR) from a provider, and your Tc is 125 ms..Your
router will divide this one sec into 8 intervals (each a Tc long) and in
that Tc you will be only sending Bc (what is allowed to send in one Tc).
However note that to send the Bc you will require only a part of the Tc for
instance half. Thus what is actually happening is you are sending for half a
Tc, and putting your interface off for another half of the Tc with the aim
of getting an average of 2Mbps.

Calculation Wise: 2Mbps/1 sec = Bc/Tc=CIR/1sec thus you can calculate the
Bc.
Then check how much time it takes you to send the Bc (based ont he interface
speed).
time required= Bc/(Access Rate of interface) = (this time will be definitely
less than Tc)

Thus in every Tc interval you will be ON for this time required/off for the
remaining time. Note that for VOIP applications it is advised that the Tc be
small so that your interface doesn't turn off for a long time and break the
150ms rule.

Regarding Be, is in fact what happens if you exceed the rate that your ISP
has given you..It happens in a way that suppose for one of the Tc intervals,
you undersubscribed your utilization, i.e. you didn't reach your Bc limit,
the provider might be kind enough to keep you some balance, and in another
time you may be allowed to burst above Bc. Thus in one Tc it might be
possible that you send (Bc + Be) the Be being the access in your account.
Same formulas/same concept with CIR but this time with PIR. Supposing that
the ISP tells you I am comitted to serve you 2Mbps but at times you may be
allowed to get a peak PIR 3Mbps.

Hope this helps..I guess for more information you can refer to INE's blog as
Petr's posts are great.
http://blog.ine.com/page/3/?s=QOS+Shaping

Best Regards,

On Wed, Aug 18, 2010 at 9:52 PM, masroor ali <masror.ali_at_gmail.com> wrote:

> hi,
>
> Can anyone explain in easy wordings that what is Bc Be and Tc
>
> --
> Regards,
> Masroor Ali
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html
>
>
>
>
>
>
>
>

-- 
KJ
Blogs and organic groups at http://www.ccie.net